Continuous flow synthesis and post-synthetic conversion of single-crystalline covalent organic frameworks

Abstract

The synthesis and scale-up of high quality covalent organic frameworks (COFs) remains a challenge due to slow kinetics of the reversible bond formation and the need for precise control of reaction conditions. Here we report the rapid synthesis of faceted single crystals of two-dimensional (2D) COFs using a continuous flow reaction process. Two imine linked materials were polymerized to the hexagonal CF-TAPB-DMPDA and the rhombic CF-TAPPy-PDA COF, respectively. The reaction conditions were optimized to produce single crystals of micrometer size, which notably formed when the reaction was cooling to room temperature. This indicated a growth mechanism consistent with the fusion of smaller COF particles. The optimized conditions were used to demonstrate the scalability of the continuous approach by synthesizing high quality, faceted COFs at a rate of more than 1 g h⁻¹. The materials showed high crystallinity and porosity with surface areas exceeding 2000 m² g⁻¹. Additionally, the versatility of the continuous flow reaction approach was demonstrated on a post-synthetic single crystal to single crystal demethylation of CF-TAPB-DMPDA to afford a hydroxyl functionalized COF CF-TAPB-DHPDA. Throughout the modification process, the material maintained its hexagonal morphology, crystallinity, and porosity. This work reports the first example of synthesizing and post-synthetically modifying imine linked COF single crystals in continuous flow and will prove a first step towards scaling high quality COFs to industrial levels.